Optical fiber cables and modules with modal disperson compensation

ABSTRACT

An optical fiber cable for bi-directional communication of optical signals is disclosed. The optical fiber cable has a primary multimode optical fiber with an alpha value of α and is concatenated to a compensating fiber with an alpha value α, wherein 1.2≤(α′−α)≤−0.1. The optical fiber cable has a reach between 50 m and 800 m. Modules that employ a plurality of concatenated primary and compensating optical fibers are also disclosed. A bi-directional optical fiber communications system that operates at two different wavelengths is also disclosed.

PRIORITY APPLICATIONS

This application is a continuation of International Application No.PCT/US14/64272, filed Nov. 6, 2014, which claims the benefit of priorityto U.S. Application No. 61/909,530, filed Nov. 27, 2013, bothapplications being incorporated herein by reference.

FIELD

The present disclosure relates to optical fiber cables, and inparticular to optical fiber cables with modal dispersion compensation.

BACKGROUND

Optical fibers (sometimes referred to herein simply as “fibers”) areused to transmit optical signals for a variety of differentapplications. Optical fibers, including multimode optical fibers, arefrequently used for data transmission over distances ranging from ameter or less (e.g., between telecommunication devices in atelecommunications center) up to distances of many meters (e.g., as longas several hundred meters or more), such as associated with transmittingoptical signals within a local-area network either within a building orbetween buildings.

Multimode fibers, by definition, are designed to support multiple guidedmodes at a given wavelength. The bandwidth of a multimode fiber isdefined by the fiber's ability to carry the different optical (guided)modes with little or no temporal separation as they travel down thefiber. This requires that the group velocities of the different opticalmodes be as close to the same value as possible. That is to say, thereshould be minimal modal dispersion (i.e., the difference in the groupvelocity between the different guided modes should be minimized) at adesign (“peak”) wavelength λ_(p).

Multimode fibers may be classified by the ISO 11801 standard as OM1,OM2, OM3 and OM4, based on the modal bandwidth of the multimode fiber.The OM4 fiber supports 150 meter links at 40 Gb/s and 100 Gb/s inaccordance with IEE 802.3ba guidance. The letters “OM” stand for opticalmultimode.

Recently, there has been interest in using optical signals of differentwavelengths to perform bi-directional (BiDi) optical signal transmissionover a multimode fiber (MM fiber) as a cost-effective way to increasethe capacity of optical fiber links for data center applications.Unfortunately, the nature of present-day MM fibers limits the range ofBiDi systems to about 100 m at the data rate of 20 Gb/s due to modaldispersion that arises by using optical signals of wavelengths that aresubstantially different from the peak wavelength λ_(p). Yet, there wouldbe great benefit to extending the reach of the BiDi cabling to beyondthe 100 m limit, e.g., to up to 150 m or even beyond.

SUMMARY

An aspect of the disclosure is an optical fiber cable for bi-directionalcommunication of optical signals. The cable includes: a primarymultimode optical fiber having first and second ends separated by alength L1, an alpha parameter α of about 2.1 and a peak wavelength λ_(p)in the range from 820 nm to 880 nm, and a first modal bandwidth BW₄₀ of4 GHz·km or greater; a compensating multimode optical fiber having firstand second ends separated by a length L2<L1 and that is opticallycoupled to the primary multimode optical fiber at the respective secondends, wherein the compensating multimode optical fiber has an alphaparameter α′<α, and wherein −1.2≤(α′−α)≤−0.1; a first optical fiberconnector operably arranged at the first end of the primary multimodeoptical fiber; a second optical fiber connector operably arranged at thefirst end of the compensating multimode optical fiber; and wherein theoptical fiber cable has a reach 50 m<LR<800 m.

Another aspect of the disclosure is a bi-directional optical fibercommunication system that uses the optical fiber cable as describedabove. The system includes: a first transceiver having a first port thatis operably connected to the first optical fiber connector of theoptical fiber cable; a second transceiver having a second port that isoperably connected to the second optical fiber connector of the opticalfiber cable; and wherein the first transceiver emits optical signals ofa first wavelength λ_(A) that travel to the second transceiver, andwherein the second transceiver emits optical signals of a secondwavelength λ_(B)≠λ_(A) that travel to the first transceiver.

Another aspect of the disclosure is a compensation module for use with aprimary optical fiber cable. The compensation module includes: a housinghaving first and second ends and an interior; at least one firstconnection port at the housing first end; at least one second connectionport at the housing second end; and a plurality of compensating opticalfibers of different length that optically connect the at least one firstconnection port to the at least one second connection port, wherein eachcompensating optical fiber provides a select amount of modal dispersioncompensation.

Another aspect of the disclosure is bi-directional optical fibercommunication system. The system includes: first and second transceivershaving respective first and second ports, wherein the first transceiveremits first optical signals of a first wavelength λ_(A) in the range 820nm≤λ_(A)≤880 nm, and wherein the second transceiver emits second opticalsignals of a second wavelength λ_(B) in the range from 880 nm≤λ_(B)≤1600nm; an optical fiber cable having respective first and second endsoptically connected to the first and second transceiver respectively,the optical fiber cable having a primary multimode optical fiber oflength L1 concatenated with a compensating multimode optical fiber oflength L2<L1; wherein the primary multimode optical fiber has an alphaparameter α of about 2.1 and a peak wavelength λ_(p) in the range from840 nm to 860 nm, and a first modal bandwidth BW₄₀ of 4 GHz·km orgreater and wherein the compensating multimode optical fiber has analpha parameter α′<α, and wherein −0.9≤(α′−α)≤−0.1; and wherein theoptical fiber cable has a reach LR=L1+L2, wherein 100 m<LR≤150 m, and amodal bandwidth BW₄₀>2100 MHz·km for both λ_(A) and λ_(B), and whereinthe primary optical fiber has a modal bandwidth BW₁<2100 MHz·km for atleast one wavelength in the range 880 nm λ_(B)≤1600 nm.

Additional features and advantages are set forth in the DetailedDescription that follows, and in part will be readily apparent to thoseskilled in the art from the description or recognized by practicing theembodiments as described in the written description and claims hereof,as well as the appended drawings. It is to be understood that both theforegoing general description and the following Detailed Description aremerely exemplary, and are intended to provide an overview or frameworkto understand the nature and character of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding, and are incorporated in and constitute a part of thisspecification. The drawings illustrate one or more embodiment(s), andtogether with the Detailed Description serve to explain principles andoperation of the various embodiments. As such, the disclosure willbecome more fully understood from the following Detailed Description,taken in conjunction with the accompanying Figures, in which:

FIGS. 1A and 1B are schematic diagrams of two examples of abi-directional optical communication system according to the disclosure.

FIG. 2 is a plot of the modal bandwidth BW₁ (GHz·km) versus wavelength λ(nm) showing three different curves (C1, C2 and C3) obtained based onmeasurements taken from an example OM4 MM fiber, and illustrating hownot all OM4 MM fibers can satisfy a modal bandwidth requirement of 2.1GHz·km at 918 nm.

FIG. 3 is similar to FIG. 2 except that the data presented is based onmodeling rather than actual measurements, with the plot showing threemembers of a distribution of one-thousand OM4 MM fibers.

FIG. 4 is a plot of the coupling offset d (μm) versus the relative delayRD (picoseconds or ps) and shows the measured differential mode delay(DMD) for a) an OM4 MM fiber primary fiber of length L1=547 m andλ_(p)=850 (square data points); b) a low-alpha compensating fiber oflength L2=230 m (circle data points), and c) an optical fiber linkformed by combining the example primary and compensating fibers(triangle data points), wherein all of the DMD measurements were takenat λ=1060 nm.

FIG. 5 is a plot of the fiber radius r (μm) versus the modal delaychange ΔC (ns/km) over a 25 nm wavelength change for the example 1 kmOM4 MM fiber primary fiber.

FIG. 6 and FIG. 7 are plots of the coupling offset d (μm) versus therelative delay RD (ns/km) that show the centroid delays for twodifferent low-alpha compensating fibers that have targeted alpha valuesof α=1.85 and 1.55, respectively.

FIG. 8 plots of relative delay (ns/km) vs. mode group number N_(MG) fordifferent values of Δα.

FIG. 9 is similar to FIG. 3 and plots the modeled bandwidths BW₄₀ for anexample optical fiber link formed from the 100 m of the OM4 MM fiber ofFIG. 3 and 5 m of compensating fiber optimized for 1300 nm.

FIGS. 10A and 10B are plots of bandwidth B (GHz) versus wavelength λ(nm) for 1 km of the primary fiber of example EX1 (see Table 1)concatenated with 20 m of a low-alpha compensating fiber.

FIG. 11 is a plot of the relative refractive index Δ(%) versus radius rfor an example bend-insensitive primary optical fiber.

FIG. 12A illustrates an example embodiment of the optical fiber cable inthe form of an LC jumper cable having LC connectors.

FIG. 12B illustrates an example embodiment of the optical fiber cable inthe form of an MPO-to-LC harness or fan-out cable with multiple LCconnectors.

FIG. 13A through FIG. 13C are elevated (FIG. 13A) and top-down partialcut-away views (FIGS. 13B, 13C) that illustrate examples of acompensation module that includes different lengths of compensatingfiber and that allow for selecting a required amount of modal dispersioncompensation.

DETAILED DESCRIPTION

Reference is now made in detail to various embodiments of thedisclosure, examples of which are illustrated in the accompanyingdrawings. Whenever possible, the same or like reference numbers andsymbols are used throughout the drawings to refer to the same or likeparts. The drawings are not necessarily to scale, and one skilled in theart will recognize where the drawings have been simplified to illustratethe key aspects of the disclosure.

The claims as set forth below are incorporated into and constitute partof this detailed description.

The entire disclosure of any publication or patent document mentionedherein is incorporated by reference.

Definitions

The term “relative refractive index,” as used herein, is defined as:Δ(r)=[n(r)² −n _(REF) ²)]/2n ₀ ²,where n(r) is the refractive index at radius r, unless otherwisespecified. The relative refractive index is defined at 850 nm unlessotherwise indicated, but could be defined at the system or operatingwavelength, λ_(s). In one aspect, the reference index n_(REF) is silicaglass. In another aspect, the system wavelength is 850 nm and n_(REF) is1.4525. In another aspect, n_(REF) is the maximum refractive index ofthe cladding. n₀ is the maximum index of the index profile. In mostcases, the n₀=n(0). As used herein, the relative refractive index isrepresented by Δ and its values are given in units of “%,” unlessotherwise specified. In cases where the refractive index of a region isless than the reference index n_(REF), the relative refractive index isnegative and is referred to as having a depressed region or depressedindex, and the minimum relative refractive index is calculated at thepoint at which the relative index is most negative, unless otherwisespecified. In cases where the refractive index of a region is greaterthan the reference index n_(REF), the relative refractive index ispositive and the region can be said to be raised or to have a positiveindex.

The alpha parameter α as used herein relates to the relative refractiveindex Δ, which is in units of “%,” where r is the radius (radialcoordinate) of the fiber, and which is defined by:

${{\Delta(r)} = {\Delta_{0}\left\lbrack {1 - \left( \frac{r - r_{m}}{r_{0} - r_{m}} \right)^{\alpha}} \right\rbrack}},$where r_(m) is the point where Δ(r) is the maximum Δ₀, r₀ is the pointat which Δ(r) % is zero and r is in the range r_(i)≤r≤r_(f), where Δ(r)is defined above, r_(i) is the initial point of the α-profile, r_(f) isthe final point of the α-profile and α is an exponent that is a realnumber. For a step index profile, α>10, and for a gradient-indexprofile, α<5. It is noted here that different forms for the core radiusr₀ and maximum relative refractive index Δ₀ can be used withoutaffecting the fundamental definition of Δ. For a practical fiber, evenwhen the target profile is an alpha profile, some level of deviationfrom the ideal situation can occur. Therefore, the alpha parameter α fora practical fiber is obtained from a best fit of the measured indexprofile. An alpha parameter α=2.1 provides a minimum for thedifferential mode delay (DMD) at 850 nm=850 nm for a multimode fiberwith Δ₀=1% made by the outside vapor deposition (OVD) process. The alphaparameter for multimode fibers made other processes such as modifiedchemical vapor deposition (MCVD) and plasma chemical vapor deposition(PCVD) can be slightly different. The alpha parameter α as used below isfor the primary fiber and the alpha parameter α′ is used for thecompensating fiber, with α>α′. The term Δα=α′−α represents thedifference between the two alpha parameters.

The limits on any ranges cited herein are considered to be inclusive andthus to lie within the range, unless otherwise specified.

The numerical aperture or NA of an optical fiber means the numericalaperture as measured using the method set forth in IEC-60793-1-43 (TIASP3-2839-URV2 FOTP-177) titled “Measurement Methods and Test Procedures:Numerical Aperture”.

The term “dopant” as used herein refers to a substance that changes therelative refractive index of glass relative to pure undoped SiO₂. One ormore other substances that are not dopants may be present in a region ofan optical fiber (e.g., the core) having a positive relative refractiveindex Δ.

The term “mode” is short for a guided mode or optical mode. A multimodeoptical fiber means an optical fiber designed to support the fundamentalguided mode and at least one higher-order guided mode over a substantiallength of the optical fiber, such as 2 meters or longer.

The aforementioned peak wavelength λ_(p) is the wavelength at which aparticular optical fiber has the highest modal bandwidth BW. The systemwavelength λ_(s) is the wavelength at which the fiber is operating andis not necessarily the peak wavelength. For example a multimode fibercan have a peak wavelength λ_(p)=850 nm but the light traveling thereincan have a system wavelength λ_(s) of 852 nm. The peak wavelength forthe primary fiber is denoted λ_(p) while the peak wavelength of thecompensating fiber is denoted λ′_(p).

The overfilled bandwidth of an optical fiber is denoted BW and isdefined herein as using overfilled launch conditions at 850 nm accordingto IEC 60793-1-41 (TIA-FOTP-204), Measurement Methods and TestProcedures: Bandwidth. The minimum calculated effective modal bandwidthEMBc can be obtained from the measured DMD spectrum as specified by IEC60793-1-49 (TIA/EIA-455-220), Measurement Methods and Test Procedures:Differential Mode Delay. The units of bandwidth for an optical fiber canbe expressed in MHz·km, GHz·km, etc., and a bandwidth expressed in thesekinds of units is also referred to in the art as the bandwidth-distanceproduct. The modal bandwidth is defined in part by modal dispersion. Atthe system level, the overall bandwidth can also be limited by chromaticdispersion, which limits the system performance at a high bit rate orfor longer system reach.

The term bandwidth is denoted by B and has units of frequency, e.g.,GHz.

The term “modal dispersion” or “intermodal dispersion” is, in an opticalfiber, a measure of the difference in the travel times of the differentmodes of an optical fiber for light of a single wavelength and isprimarily a function of the alpha profile of the optical fiber.

BiDi Optical Communication System

FIGS. 1A and 1B are schematic diagrams of two examples of abi-directional optical communication system (“BiDi system”) 10 accordingto the disclosure. The BiDi system 10 includes two BiDi transceivers 20Aand 20B with respective ports 22A and 22B. Transceivers 20A and 20B areoptically connected by an optical fiber cable 30 that has end connectors32A and 32B that are connected to respective transceiver ports 22A and22B. Transceivers 20A and 20B respectively emit optical signals 24A and24B that have respective wavelengths λ_(A) and λ_(B). In an exampleembodiment, 820 nm≤λ_(A)≤880 nm and 880 nm≤λ_(B)≤1600 nm. In otherexample embodiments, 880 nm≤λ_(B)≤1360 nm or 880 nm≤λ_(B)≤1100 nm or 880nm≤λ_(B)≤1000 nm.

In the example BiDi system 10 of FIG. 1A, optical fiber cable 30includes a single optical fiber link 40 of length LR that includes afirst or primary multimode optical fiber section (“primary fiber”) 44 oflength L1 and having first and second ends 46 and 48. An example primaryfiber 44 has an alpha parameter α of about 2.1, a peak wavelength λ_(p)in the range from 840 nm to 860 nm, and a modal bandwidth BW1≥4.7GHz·km. An example multimode fiber with these properties is called OM4multimode fiber.

Primary fiber 44 is operably connected at its first end 46 totransceiver 20A and at its second end 48 to a second or compensatingmultimode optical fiber section (“compensating fiber”) 54 of length L2,which is described in greater detail below. Compensating fiber 54 has anend 56 operably connected to transceiver 20B and an end 58 operablyconnected to end 48 of primary fiber 40. Thus, optical fiber link 40comprises concatenated primary and compensating fibers 44 and 54. In anexample, the primary and compensating optical fibers 44 and 54 areencased in a jacket 60. Thus, an exemplary optical fiber cable 30includes optical fiber link 40 formed by primary and compensating fibers44 and 54, jacket 60, and end connectors 32A and 32B at respective endsof the optical fiber link, wherein the end connectors are configured tooperably connect with a mating connector or with a device port.

In an example, a connector 62 is used to optically connect primary andcompensating optical fibers 44 and 54 at their respective ends 48 and58. The primary and compensating optical fibers 44 and 54 can also bespliced together at their respective ends 48 and 58 using known opticalfiber splicing techniques. In an example, primary fiber 44 is an OM4 MMfiber, which is specified at 850 nm (i.e., has a peak wavelength) λ_(p)of about 850 nm) and that has an alpha parameter α of about 2.1. Themodal bandwidth of primary fiber 44 is denoted BW₁, the modal bandwidthof compensating fiber 54 is denoted BW₂, and the modal bandwidth ofoptical fiber link 40 is denoted BW₄₀, which defines the modal bandwidthof BiDi system 10. In the discussion below, the peak wavelength λ_(p)refers to primary fiber 44 unless otherwise indicated.

The example BiDi system 10 of FIG. 1B shows optical fiber cable 30 ashaving two optical fiber links 40. An example of such an optical fibercable 30 is an LC duplex cable. For such a dual-link optical fiber cable30, transceivers 20A and 20B transmit and receive over each opticalfiber link 40 (e.g., 20 GbE or 50 GbE over each optical fiber link, fora total of 40 GbE or 100 GbE in each direction). In general, opticalfiber cable 30 can comprise one or more optical fiber links 40, withsuitable end connectors 32A and 32B (e.g., LC duplex connectors or MPOconnectors).

In an example, transceivers 20A and 20B are quad small-form-factorpluggable (QSFP+) transceivers that respectively include vertical-cavitysurface-emitting laser (VCSEL) light sources 27A and 27B. The BiDisystem 10 has a reach, i.e., the aforementioned length LR=L1+L2 betweentransceivers 20A and 20B. In an example, 1 m<LR≤800 m, and further in anexample is 1 m<LR≤150 m, and further in an example is LR is nominally150 m.

As noted above, transceivers 20A and 20B of BiDi system 10 transmitoptical signals 24A and 24B over each optical fiber link 40 in oppositedirections but at different wavelengths λ_(A) and λ_(B), e.g., λ_(A)=850nm and λ_(B)=900 nm. The different wavelengths serve to reducereflections at the opposite transceiver. Further, in an example,transceivers 20A and 20B have respective wavelength filters 25A and 25Bthat allow for transmission of their respective wavelengths λ_(A) orλ_(B) while blocking the other wavelength.

In an example, the modal bandwidth BW₄₀ of optical fiber link 40 is inthe range from 2500 MHz·km to 2800 MHz·km at nominally=850 nm. The modalbandwidth of the primary fiber varies as a function of wavelength λ, andin particular gets smaller with increasing wavelength. A long-wavelengthVCSEL light source 27 can have a center wavelength of 900 nm with arange of +/−18 nm for the entire wavelength distribution (spectrum).Therefore, as a worst case, the bandwidth BW₄₀ of optical fiber link 40needs to satisfy the minimum bandwidth requirement at λ=918 nm. In anexample based on industry needs, the modal bandwidth BW₄₀ for a reachLR=150 m at 900 nm+/−10 nm needs to be between 2100 MHz·km and 2500MHz·km.

While OM4 MM fibers are desirable for use in BiDi systems, not all OM4MM fibers can meet the modal bandwidth requirement of BW₄₀=2100 MHz·kmfor a reach LR=150 m at the wavelength within the range from 882 nm to918 nm. FIG. 2 is a plot of the modal bandwidth BW₁ (GHz·km) versuswavelength λ (nm) showing three different curves (C1, C2 and C3)obtained based on measurements taken from example OM4 MM fibers. The OM4modal bandwidth threshold of 4.7 GHz·km is shown, along with the 2.1GHz·km threshold. The vertical dashed lines correspond to the maximum ofthe respective curves and thus the respective peak wavelengths on thehorizontal wavelength axis.

The modal bandwidth measurements were made using a ModCon modecontroller (available from Arden Photonics Ltd., West Midlands, UnitedKingdom) to regulate the launch condition, which provides a typicalVCSEL launch condition but with coverage over most of the radial region.The fitting function used for the modal bandwidth curves had thefollowing form:

${BW} = \frac{0.2}{\sqrt{a^{2} + {c^{2}\left( {\lambda - \lambda_{p}} \right)}^{2}}}$where a and c are curve-fitting parameters. Note that λ=λ_(p)corresponds to the maximum modal bandwidth value.

The information from FIG. 2 can be used to analyze the capability of OM4MM fibers at 918 nm because the OM4 MM fibers with highest peak modalbandwidth BW₁ and lowest peak wavelength λ_(p) (Curve C1) sets the lowerbound on the modal bandwidth BW₁ for OM4 fiber at 918 nm. As curves C1,C2 and C3 have the same basic form, the curves can be shifted to theleft-most peak wavelength λ_(p) and to the right most peak wavelengthλ_(p) while keeping the modal bandwidth BW₁ at λ=850 nm above 4700MHz·km.

The range on the peak wavelength λ_(p) can vary from about 820 nm toabout 880 nm, and in an example ranges from 840 nm to 860 nm. The 820 nmto 880 nm range should be very close to the actual wavelength range forthe peak wavelength of OM4 MM fiber for the whole distribution. An OM4MM fiber with a peak modal bandwidth near 820 nm can result in bandwidthBW₁ at 918 nm lower than 2.1 GHz·km requirement. But, for OM4 MM fiberswith sufficiently high peak wavelength λ_(p), such as those above 850nm, the modal bandwidth BW₁ at 918 nm is above 2.1 GHz·km.

The above analysis is supported by numerical modeling of OM4 MM fibers.FIG. 3 is a plot similar to FIG. 2 but is based on modeled modalbandwidth BW₁ versus wavelength λ for three members of a modeleddistribution of one-thousand MM fibers. The modeling used variations inthe alpha parameter to generate OM4 MM fibers with a range of peakwavelengths λ_(p). Only those members of the distribution meeting theOM4 MM fiber bandwidth BW₁ at 850 nm are shown. The modal bandwidth BW₁was calculated based on an overfill launch condition.

The curves C1′ C2′ and C3′ of FIG. 3 are consistent with the curves C1,C2 and C3 of FIG. 2. In particular, the modeled modal bandwidth of theOM4 MM fibers of FIG. 3 show that OM4 MM fibers with peak modalbandwidths BW₁ at a peak wavelength λ_(p) lower than about 840 nm willnot be able to exceed 2.1 GHz·km at λ=918 nm.

Consequently, optical fiber link 40 is compensated for modal dispersionby employing compensating fiber 54 so that any OM4 MM fiber can be usedas primary fiber 44, thereby enabling the optical fiber link can have areach LR in excess of 100 m, e.g., 150 m.

To illustrate modal dispersion compensation effect using a compensationfiber, FIG. 4 is a plot of the coupling offset d (μm) versus relativedelay RD (picoseconds or ps) and shows the measured DMD for a) anexample OM4 MM fiber primary fiber 44 of length L1=547 m and peakwavelength λ_(p)=850 nm (square data points), b) for a low-alphacompensating fiber of length L2=230 m (circle data points), and c) theoptical fiber link 40 formed by combining the example primary andcompensating fibers (triangle points). All of the DMD measurements ofFIG. 4 where taken at λ=1060 nm. The DMD centroid for the OM4 MM fiberprimary fiber 44 is straight within 100 ps over a 1 km length atλ_(p)=850 nm, but at λ=1060 nm it is right-tilted due to materialdispersion. On the other hand, the low-alpha compensating fiber 54 has aleft-tilted DMD centroid at λ=1060 nm. Thus, the DMD curve for theoptical link 40 that combines the primary and compensating fibers 44 and54 is largely centered about RD=0 ps. In this example, the peakwavelength of the total link is moved from 850 nm of the primary fiberto 1060 nm. By changing the alpha value and the length of thecompensation fiber, the peak wavelength of the total link can be movedto any desired wavelength, for example, 850 nm, 870 nm, 890 nm or 910nm.

Optical fiber link 40 needs to have a certain degree of modal dispersioncompensation to achieve the desired reach LR for the given opticalsignals 40A and 40B of wavelengths λ_(A) and λ_(B), e.g., for λ_(A)=850nm and λ_(B)=918 nm. Note that at λ_(A)=850 nm, the required modalbandwidth BW₄₀ for a reach of LR=150 m is in the range 2500 2800 MHz·kmand at λ_(B)=918 nm, the modal bandwidth BW₄₀ needs to be in the range2100-2500 MHz·km.

Based on the information in FIGS. 2 and 3, the peak wavelength λ_(p) forthe OM4 MM fiber 44 needs to be shifted upwards by 15 nm to 30 nm toboost the 918 nm modal bandwidth BW₄₀ above the 2.1 GHz·km threshold,while keeping the 850 nm bandwidth BW₄₀ above the 2500 MHz·km threshold.For example, if curve C1 of FIG. 2 is shifted to the right by 25 nm,then the bandwidth BW₄₀ will be above 2.1 GHz·km, and perhaps even above2.5 GHz·km. On the other hand, the 850 nm modal bandwidth BW₄₀ would goup farther from the OM4 MM fiber threshold level of 4.7 GHz·km, which isbetter.

The impact of moving curves C2 and C3 to the right by 25 nm isdifferent. In particular, for curve C3, the 918 nm modal bandwidth BW₄₀is far more than needed while the 850 nm bandwidth BW₄₀ decreases, butstill has a value around 2700 MHz·km and is within the range of themodal bandwidth BW₄₀ needed. To optimize the performance of BiDi system10, optical link 40 can employ different lengths L2 of compensatingoptical fiber 54 for different OM4 MM fiber s primary fibers 44 such asrepresented by curves C1, C2 and C3 of FIG. 2. A similar analysis can bedone using modeling-based results, such as presented in FIG. 3.

From the above analysis, in one example embodiment the goal of the modaldispersion compensation in BiDi system 10 is to shift the peakwavelength λ_(p) of the primary fiber 44 up by 20 nm to 30 nm, e.g., byabout 25 nm. Said differently, the goal is to form an optical fiber link40 wherein the peak wavelength λ_(p40) of the link is shifted up by 20nm to 30 nm relative to the peak wavelength λ_(p) of primary fiber 44.

By way of example, the wavelength dependence of the modal dispersion ofan example OM4 MM fiber primary fiber 44 with λ_(p)=850 nm isconsidered. The modal delay change ΔC (in unit of ns/km) with radius rof the DMD centroid can be described by the equationΔC(r)=[Δλ/D]·(r/a)², where r is a radial coordinate of the OM4 MM fiber44, Δλ is the change (in nm) of wavelength from 850 nm, and a is thecore radius. Over a wavelength range of 30 nm to 40 nm around 850 nm, Dis a coefficient with the value of about 300 (nm·km/ns). The equationfor ΔC was obtained by modeling the DMD behavior of 1 km of OM4 MM fiberhaving an alpha refractive index profile at wavelengths around 850 nm,and fitting the centroid difference in the wavelength regime.

FIG. 5 is a plot of the fiber radius r (μm) versus the modal delaychange ΔC (ns/km) and shows the amount of modal delay change over a 25nm wavelength change for the example 1 km OM4 MM fiber primary fiber 44.The amount of modal delay needed at a given wavelength (say 850 nm) toshift the peak wavelength λ_(p) by 25 nm would be −ΔC. For 150 m of OM4MM fiber, to shift the peak wavelength λ_(p) up by 25 nm, the modaldelay change needed is ΔC=(83/1000)·150=12.5 ps at r=25 μm.

FIG. 6 and FIG. 7 are plots of the coupling offset d (μm) versus therelative delay RD (ns/km) that show the centroid delays for twodifferent low-alpha compensating fibers 54 with targeted alpha values ofα′=1.85 and 1.55, respectively. These are referred to as “low alpha”fibers because the values of the alpha parameter α′ are smaller than thevalue of α=2.1 that is known to minimize DMD at 850 nm. The two examplelow-alpha compensating fibers 54 have a delay (per meter) of 1.8 ps and4.3 ps, respectively. This means that to shift the peak wavelength λ_(p)of 150 m of OM4 MM fiber up by 12.5 ps, the compensating fiber withα′=1.85 alpha needs to have a length of L2=7 m, or for α′=1.55 needs tohave a length L2=2.9 m. It is feasible to make compensating fibers 54with even smaller alpha parameters α′ because the delay scales with theΔα. This scaling is shown in FIG. 8, which plots of relative delay(ns/km) vs. mode group number N_(MG) for different values of Δα. In anexample, −1.2≤Δα≤0.1. Also in an example, compensating fiber has a peakwavelength λ′_(p)>880 nm, a modal bandwidth BW₂<500 MHz·km, and amaximum relative refractive index Δ₀, wherein 0.7%≤Δ₀≤3.0% or in anotherembodiment to be 0.8%≤Δ₀≤1.5% or in another embodiment 0.9%≤Δ₀<1.2%.

In an example, compensation fiber 54 can be a MM fiber optimized foroperation at relatively long wavelengths, e.g., 1300 nm. In such a case,the length L2 of compensating fiber 54 may be longer than a compensatingfiber optimized at a shorter wavelength. Such a long-wavelengthcompensating fiber 54 may represent a substantial portion of opticalfiber link 40 and thus may not be suitable for use as jumpers. In thiscase, the compensation fiber can be part of the total cable link.

FIG. 9 is similar to FIG. 3 and plots the modeled bandwidths BW₄₀ for anexample optical fiber link 40 formed from 100 m of OM4 MM fiber 44 ofFIG. 3 and 5 m of compensating fiber 54 optimized for 1300 nm. Addingthe compensating fiber 54 serves to shift the curves of FIG. 9 to longerwavelengths by about 15 nm. However, this is not quite enough toguarantee that the worst case OM4 MM fiber will allow optical fiber link40 to meet the bandwidth requirement for BW₄₀ at 918 nm for reach LR.Thus, the length L2 of the compensating fiber 54 should be increased,e.g., to be in the range from 7 m to 10 m.

Experimental Results

Experiments were performed to validate the effectiveness of the modaldispersion compensation of optical fiber link 40 and optical fiber cable30. Three example primary fibers 44 with λ_(p) less than 830 nm wereused to illustrate the modal dispersion compensation. The fiber lengthL1 for each example fiber 44 was essentially 1 km (i.e., 999.5 m).

The bandwidths BW₁ of the example primary fibers 44 at variouswavelengths around 850 nm and at 914 nm were measured using a ModConmode conditioner. Because of the relatively low peak wavelength λ_(p),the bandwidth BW₁ of each of the three example primary fibers 44 asmeasured at 918 nm were around or below the threshold modal bandwidth of2100 MHz·km needed for 20 GHz transmission over a reach of LR=150 m.

Further in the experiments, an example compensating fiber 54 of lengthL2=20 m and α′=1.55 was fabricated. The example primary fibers 44 werethen connected to the example compensating fiber 54 to form threecorresponding example optical fiber links 40. Table 1 below sets for thevarious parameters for the example primary fibers 44 (denoted EX1, EX2and EX3), and the corresponding optical fiber links 40.

TABLE 1 BW₁ @ BW₁ @ 850 nm 914 nm BW₄₀ @ BW₄₀ @ PRIMARY L1 λ_(p) (GHz ·(GHz · 850 nm 914 nm FIBER (m) (nm) km) km) (GHz) (GHz) EX1 999.5 8275.35 2.12 6.94 3.7 EX2 999.5 828 4.07 2.13 4.42 2.97 EX3 999.5 806 3.631.8 5.27 2.85

The bandwidth BW₄₀ of the three example optical fiber links 40 wasincreased to above 2.8 GHz·km (relative to bandwidth BW₁ of primaryfiber 44), which is more than enough to meet the bandwidth requirementfor a reach of LR=150 m. The bandwidth BW₄₀ at 850 nm also increased asλ_(p) increased.

FIGS. 10A and 10B are plots of bandwidth B (GHz) versus wavelength λ(nm). FIG. 10A is for the example EX1 primary fiber 44, while FIG. 10Bis for the optical fiber link 40 formed from example EX1 primary fiber44 and the 20 m low-alpha compensating fiber 54. The solid-line curvesrepresent a best-fit to the data. Note that for a 150 m BiDiapplication, one only needs L2=3 m of compensating fiber 54. The plotsof FIGS. 10A and 10B show a shift in λ_(p) of about 25 nm.

In an example, primary fiber 44 is bend insensitive, e.g., itsrefractive index profile Δ(%) has a low index trench structure in thecladding as a function of fiber radius r such as shown in FIG. 11, sothat optical fiber link 40 can be deployed in more challengingenvironments or compact spaces. In an example, embodiment, compensatingfiber 54 is also bends insensitive. The example refractive index profileΔ(%) includes regions with Δ₂, Δ₃ and Δ₄, wherein the region associatedwith Δ₃ between radii r₂ and r₃ is a trench region wherein Δ₃<Δ₂, Δ₄.Examples of bend-insensitive optical fibers are disclosed in U.S. PatentApplication Publication No. 2012/0230638 and in U.S. Pat. No. 8,406,952.

In example embodiments, optical fiber link 40 (and thus optical fibercable 30) has a modal bandwidth BW₄₀>5 GHz·km or BW₄₀>7 GHz·km in thewavelength range from 880 nm to 1600 nm. Also in example embodiments,optical fiber link 40 and optical fiber cable 30 support thetransmission of optical signals 24A and 24B at a data rate of 10 Gb/s orgreater, or at a data rate of 16 Gb/s or greater, or at a data rate of25 Gb/s or greater.

Optical fiber cable 30 can have a variety of different forms. FIG. 12Aillustrates an example embodiment where optical fiber cable 30 is in theform of an LC jumper cable having LC connectors 32A and 32B. FIG. 12Billustrates an example embodiment wherein optical fiber cable 30 is inthe form of an MPO to LC harness or fan-out cable with multiple LCconnectors 32B.

FIG. 13A is an elevated view of an example compensation module 200 thatcan be used to form optical fiber links 40 where the amount of modaldispersion compensation can be selected. Compensation module 200 has ahousing 210 with a front end 212, a back end 214, a top 216 and aninterior 218. Front end 212 includes a plurality of first connectorports 222 while back end 214 includes a single second connector port224. Six first connector ports 222 are shown by way of example. In otherexamples, anywhere from two to twenty-four first connector ports 222 canbe used. The actual number of first connector ports 222 employed can belarger than twenty four, and is only limited by the physical size ofcompensation module 200. In an example, successively numbers firstconnection ports 222 correspond to successively longer compensatingoptical fibers 54.

A primary optical fiber cable 244 that includes a length L1 of primaryfiber 44 and that is terminated by an optical fiber connector 254 isoptically connected to a second connector port 224 via optical fiberconnector 254. In an example, an optical fiber cable 270 (e.g., ajumper) is optically connected to one of first connection ports 222. Anoptical switch 300 optically connects second connector port 224 to aplurality of compensating fibers 54 each having different lengths L2. Inanother embodiment, the optical switch 300 optically connects secondconnector port 224 to a plurality of compensating fibers 54 each havingsame length L2, but with difference alpha values. Optical switch 300includes a dial 302 or like device for selecting an optical connectionbetween second connector port 224 and a given first connector port 222.Thus, each first connector port 222 corresponds to a different amount ofmodal dispersion compensation. This allows a user to select a particularfirst connector port 222 to form an optical connection based on theparticular application and the needs in the field.

FIG. 13B is a top-down view of another example of compensation module200 that includes two optical switches 300A and 300B with respectivedials 302A and 302B. The use of two optical switches 300A and 300Ballows the compensation module 200 to have a single first connector port222. The user can use dials 302A and 302B to select the appropriatecompensating fiber 54 having the length L2 that provides the requiredamount of mode dispersion compensation when connecting, for example, tooptical fiber cable 270.

FIG. 13C is similar to FIG. 13B and shows an example compensation module200 that has a plurality of first connector ports 222 and a plurality ofsecond connector ports 224. First connector ports 222 are opticallyconnected to corresponding second connector ports 224 via compensatingoptical fibers 54 having different lengths L2. Compensation module 200thus allows for optical fiber cables 244 and 270 to be opticallyconnected while having select amounts of modal dispersion compensation.

In example embodiment, compensation module includes Universal polaritymanagement or Methods A, B, C polarity management according to theTIA-568-C.0 standard.

The compensation modules shown in FIGS. 13A-13C offer flexibility inselecting a desired amount modal dispersion compensation to achieveoptimum system performance. For the BiDi application with centertransmission wavelengths of λ_(A)=850 nm and λ_(B)=908 nm, the desiredpeak wavelength λ_(p) for the link 40 is about 880 nm. With this optimumpeak wavelength, the bandwidths at both λ_(A)=850 nm and λ_(B)=908 nmare about at the OM4 level, which can increase the data rate for thetransmission distance. For a typical OM4 fiber distribution with peakwavelength λ_(p) between 820 nm and 880 nm, the peak wavelength shift isbetween 0 to 60 nm. The multipart switching module design with differentcompensation jumpers allows choosing the right amount of compensation tomove the peak wavelength close to the target.

Table 2 sets forth example lengths L2 of compensation fiber 54 for afour-fiber compensation module 200. In this embodiment, the compensationfiber 54 has an alpha of α′=1.55. This example compensation module 200is designed to work with a OM4 fiber link up to 150 m.

TABLE 2 Four-fiber compensation module configuration Fiber port 1 2 3 4L2 (m) 0 2.4 4.8 7.2

Table 3 sets forth example lengths L2 of compensation fiber 54 for aneight-fiber compensation module 200. In this embodiment, thecompensation fiber 54 has an alpha of α′=1.55. This module is designedto work with a OM4 fiber link of length LR up to 300 m.

TABLE 3 Eight-fiber compensation module configuration Fiber port 1 2 3 45 6 7 8 L2 (m) 0 2.1 4.2 6.3 8.4 10.5 12.6 14.7

The fiber selection is according to the MMF length L1 and it peakwavelength

_(p). For example, if the link length L1 is 150 m, and the peakwavelength is 850 nm, the required length for compensation fiber α′=1.55is 4.53 m. We can choose port “3” in the four-port compensation module200 or port “3” in the eight-port compensation module. In anotherexample, if the link length L1 is 230 m, and the peak wavelength isλ_(p)=830 nm, the required length L2 for compensation fiber with α′=1.55is L2=9.7 m. We can choose port “6” in the eight-port compensationmodule 200.

An aspect of the disclosure is a method of performing bi-directionalcommunication of optical signals 24A and 24B over optical fiber cable30. With reference to FIG. 1, the method includes transceivers 20A and20B emitting optical signals 24A and 24B that travel over optical fibercable 30 to the opposite transducer. The optical signals are thendetected and processed by the respective transducers 20A and 20B.Filters 25A and 25B serve to block reflected optical signals 24A and 24Brespectively so that the emitted and reflected optical signals are notprocessed by the emitting transducer as a received optical signal. In anexample, optical fiber cable 30 has a bandwidth BW₄₀>2100 MHz·km foroptical signals 24A and 24B and a reach LR that could not meet theminimum bandwidth requirement at wavelengths λ_(A) and λ_(B) with anoptical fiber cable having only primary fiber 44.

It will be apparent to those skilled in the art that variousmodifications to the preferred embodiments of the disclosure asdescribed herein can be made without departing from the spirit or scopeof the disclosure as defined in the appended claims. Thus, thedisclosure covers the modifications and variations provided they comewithin the scope of the appended claims and the equivalents thereto.

What is claimed is:
 1. An optical fiber cable for bi-directionalcommunication of optical signals, comprising: a primary multimodeoptical fiber having first and second ends separated by a length L1, analpha parameter α of about 2.1 and a peak wavelength λ_(p) in the rangefrom 820 nm to 880 nm, and a first modal bandwidth BW₄₀ of 4 GH·km orgreater; a compensating multimode optical fiber having first and secondends separated by a length L2<L1 and that is optically coupled to theprimary multimode optical fiber at the respective second ends, whereinthe compensating multimode optical fiber has an alpha parameter α′<α,and wherein −1.2≤(α′−α)≤−0.1; a first optical fiber connector operablyarranged at the first end of the primary multimode optical fiber; asecond optical fiber connector operably arranged at the first end of thecompensating multimode optical fiber; and wherein the optical fibercable has a reach 50 m<LR<800 m; and wherein the compensating multimodeoptical fiber has: a) a bandwidth BW₂ of less than 500 MHz·km atnominally 850 nm; c) a peak wavelength λ′_(p)>880 nm; and b) a maximumrelative refractive index Δ₀ and wherein 0.7%≤Δ_(o)≤3.0%.
 2. The opticalfiber cable according to claim 1, wherein the compensating multimodeoptical fiber has a peak wavelength λ′_(p)>880 nm.
 3. The optical fibercable according to claim 1, wherein at least one of the primarymultimode optical fiber and the compensating multimode optical fiber isbend-insensitive.
 4. The optical fiber cable according to claim 1,wherein LR is about 150 m.
 5. The optical fiber cable according to claim1, wherein the optical fiber cable is configured as either an LC jumpercable or as an MPO-to-LC fan-out cable.
 6. The optical fiber cableaccording to claim 1, wherein L2<(0.2)·L1.
 7. The optical fiber cableaccording to claim 6, wherein L2<(0.1)·L1.
 8. A bi-directional opticalfiber communication system, comprising: the optical fiber cableaccording to claim 1; a first transceiver having a first port that isoperably connected to the first optical fiber connector of the opticalfiber cable; a second transceiver having a second port that is operablyconnected to the second optical fiber connector of the optical fibercable; and wherein the first transceiver emits optical signals of afirst wavelength 4 that travel to the second transceiver, and whereinthe second transceiver emits optical signals of a second wavelengthλ_(B)≠λ_(A) that travel to the first transceiver.
 9. The bi-directionaloptical fiber communication system according to claim 8, wherein 820nm≤λ_(A)≤880 nm and 880 nm≤λ_(B)≤1600 nm.
 10. The bi-directional opticalfiber communication system according to claim 8, wherein 100 m<LR≤150 m.11. The bi-directional optical fiber communication system according toclaim 8, wherein the optical fiber cable supports transmission ofoptical signals at a data rate of 10 Gb/s or greater.
 12. Thebi-directional optical fiber communication system according to claim 11,wherein the optical fiber cable supports transmission of optical signalsat a data rate of 16 Gb/s or greater.
 13. The bi-directional opticalfiber communication system according to claim 12, wherein the opticalfiber cable supports transmission of optical signals at a data rate of25 Gb/s or greater.
 14. The bi-directional optical fiber communicationsystem according to claim 8, wherein the first and second transceiversinclude respective first and second vertical-cavity surface-emittinglasers (VCSELs).